This invention relates to the catalyzed bromination of hydroxyaromatic compounds, and more particularly to the use of transition metal compound catalysts for improved para- and mono-selectivity of the reaction products.
Monocyclic dihydroxyaromatic compounds such as hydroquinone and dihydroxybiphenyls such as 4,4′-dihydroxybiphenyl (hereinafter sometimes simply “biphenol”) have numerous uses in the chemical industry. For example, both compounds can be used in polymer preparation, notably in the preparation of polycarbonates, polysulfones and polyimides, especially polyetherimides.
There are various methods for the preparation of hydroquinone and biphenol. As examples of such methods, each compound can be prepared from p-bromophenol: hydroquinone by hydrolysis: and biphenol by reductive coupling in the presence of a noble metal catalyst, a base and a reducing agent.
Brominated hydroxyaromatic compounds, as exemplified by p-bromophenol, can be prepared by reaction of the precursor hydroxyaromatic compound with HBr, elemental bromine, or with various kinds of bromides. Commonly assigned, co-pending U.S. application Ser. No. 10/342,475 (filed Jan. 16, 2003) discloses an efficient means of preparing brominated hydroxyaromatic compounds including p-bromophenol. The method comprises contacting a hydroxyaromatic compound with oxygen and a bromide source in an acidic medium, in the presence of elemental copper or a copper compound as catalyst.
While this approach provides additional efficiencies relative to other known methods, improved methods continue to be sought. In particular, it would be advantageous to reduce the formation of multi-brominated by-products, thereby increasing the selectively of the reaction to form predominantly mono-brominated products, especially those where substitution occurs in the para-position relative to the hydroxy group. Furthermore, because of the extremely high corrosiveness of copper salts in acid media, it would be advantageous if other catalysts could be used in the oxybromination of hydroxyaromatic compounds.